Power supplies for electric vehicles are required to have the following characteristics: large battery capacity, less degradation through cycles, available high-speed charging and discharging, etc. Especially, a lithium ion secondary battery (hereinafter referred to as a Li ion secondary battery) has attracted attention.
As a positive electrode active material for the Li ion secondary battery, lithium cobaltate produces a relatively large battery capacity of about 150 mAh/g. The lithium cobaltate, however, still has the problems associated with safety at high temperature or in overcharge due to its high activity. For this reason, the lithium cobaltate is not used for applications of the electric vehicles.
Thus, an olivine-structured lithium iron phosphate and lithium manganese phosphate, a substitutional spinel-structured lithium manganese oxide, lithium vanadium oxide, and lithium vanadate and the like have been proposed as safer material, and the improvement of the properties of such materials has been studied.
Patent Document 1 discloses a technique that uses an amorphous V2O5—P2O5 for a positive electrode active material so as to provide an entire solid lithium secondary battery with a large capacity and excellent charging and discharging cycle characteristics.
Patent Document 2 discloses a technique that sets an average crystallite size of crystals of a lithium iron phosphate or lithium manganese phosphate having an olivine crystal structure to 140 nm or less so as to obtain good charging and discharging capacities and good load characteristics.
Patent Document 3 discloses a technique that uses an electrode active material mainly containing at least one of glass and glass ceramic composed of lithium-contained iron vanadium phosphate which contains at least one element selected from the group consisting of Co, Mn and Ni so as to obtain the high discharging rate, discharging current, and battery output.
Patent Document 4 discloses a technique that uses a spinel lithium manganese oxide in which Mn is partly replaced by at least one kind of transition metal elements selected from the group consisting of Co, Ni, Fe, V, Cr and Ti so as to provide a positive electrode active material keeping good performance of a positive electrode after the continuous use thereof under a high-temperature circumstance at a charging voltage of 4 V or more.
Patent Document 5 discloses a technique that contains a combination of a crystalline phase and an amorphous phase of a composite oxide of vanadium, and lithium or a first transition metal in a positive electrode active material so as to provide a lithium secondary battery with a large charging and discharging capacity, a high energy density, and a long cycle life.